The present invention relates to a method for controlling a cruise control in a vehicle. The invention also relates to a vehicle cruise control system intended for such method for controlling said cruise control.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing said method.
Motor vehicles, such as cars, lorries, towing vehicles and buses', are often provided with a so-called cruise control system, also denominated speed control system, for automatically controlling the vehicle speed. Such a cruise control system comprises means, such as a speed sensor, for monitoring the actual vehicle speed. The cruise control system compares the actual vehicle speed with a set target speed. The target speed may for instance be entered into the cruise control system as the prevailing actual vehicle speed when a set switch is actuated by the driver. The cruise control system generates an error signal by comparing the actual vehicle speed with the target speed. The error signal is then for instance used to control an actuator coupled to the fuel pump or to the vehicle throttle in order to change the engine speed until the error signal is substantially zero, i.e. until the actual vehicle speed is equal to the target speed.
EP 1439976 and U.S. Pat. No. 6,990,401 disclose two examples of prior art where the cruise control system has been further developed. Here the cruise control system is a predictive cruise control system utilizing information about current vehicle position and upcoming road topography, that is for example gradients or elevation values for the coming road, in order to control throttle opening in such a way as to increase fuel efficiency.
A problem with prior art cruise control is that unnecessary downshifts can occur during certain vehicle driving conditions. Three examples of such vehicle driving conditions are disclosed in FIGS. 1 to 3. Each of said figures disclose a typical example of a road section profile 1, 21 and 31 where such unnecessary downshifts can occur. Road section profile 1 in FIG. 1 disclose a downhill followed by an uphill slope. An upper curve 2 discloses how the vehicle speed varies when driving said road section profile 1 and a cruise control set on target speed vcc set target speed. In the disclosed example a maximum allowable overspeed vbcc is set for when a brake cruise will activate auxiliary brakes and/or service brakes of said vehicle in order not to exceed said Vbcc when driving in a downhill. With the prevailing vehicle condition a certain gear in a transmission of said vehicle is engaged and a lower vehicle speed limit for when a downshift (vd0wnshift speed) to a lower gear (with higher gear ratio) will occur is determined. Said engaged gear can be a direct gear or a gear where torque is transmitted via gear wheel pairs in the transmission (gearbox). In FIG. 1 when the vehicle is in position A the vehicle is accelerating, and eventually accelerating up to a bit over vcc set target speed after position A. When the vehicle is in position B the uphill slope has started and the vehicle speed is decreasing. In the shown example the inclination is so steep that the available propulsive power of the vehicle is not enough to hold vcc set target speed with the current gear engaged. In position C the vehicle speed has decreased to said v downshift speed and a downshift is performed. In the shown example said downshift results in that vehicle speed starts to increase. A drawback is that fuel efficiency is lost due to propulsive power interruption during said downshift. Even more fuel efficiency will be lost if said downshift is a downshift from a direct gear, since driving with a direct gear engaged is more fuel efficient compared to a gear where torque is transmitted via gear wheel pairs.
The examples disclosed in FIGS. 2 and 3 disclose corresponding downshifts C for further two different possible road section profiles 21 and 31. Road section profile 21 starts from a horizontal road followed by an uphill slope. Road section profile 31 starts from a light uphill slope, which is followed by an uphill slope.
It is desirable to further develop such a cruise control system where information about current vehicle position and upcoming road topography is used by the cruise control for controlling vehicle speed.
It is desirable to present an improved method for cruise control which can avoid unnecessary downshifts in uphill slopes.
The method according to an aspect of the invention is a method for controlling a cruise control during driving of a vehicle. Said method comprises (includes, but is not necessarily limited to) the steps of:                driving said vehicle with said cruise control active and set to maintain a vehicle set target speed;        registering current vehicle condition, which comprises at least a current vehicle position, a currently engaged gear ratio, available gear ratios, current vehicle speed, available maximum propulsion torque and road topography of coming travelling road comprising a next coming uphill slope;        based on said current vehicle condition predicting a downshift at a coming vehicle position in said coming uphill slope due to vehicle speed decrease and selecting at least one activity which results in that said downshift can be postponed or avoided;        controlling said cruise control according to said selected activity.        
According to a first alternative embodiment of said invention said activity comprises the step of:                temporary lowering a downshift limit of said currently engaged gear with a predetermined possible amount to a lowered downshift speed limit.        
According to a further embodiment of said invention said method said activity further comprises the steps of:                predicting if said downshift will be postponed enough or avoided;        if predicting that said downshift will not be postponed enough or not avoided then performing said temporary lowering of said downshift limit;        and additionally based on said current vehicle condition, calculating a lowest vehicle speed for a first vehicle position where the vehicle will start to retard in said uphill slope and which results in that said downshift can be enough postponed or avoided;        based on said current vehicle condition controlling said cruise control in order to during vehicle driving towards said first vehicle position increase vehicle speed to said lowest vehicle speed when said vehicle eventually has reached said first vehicle position.        
In another embodiment of the invention said activity comprises the steps of:                based on said current vehicle condition calculating a lowest vehicle speed for a first vehicle position where the vehicle will start to retard in said uphill slope which results in that said downshift can be postponed or avoided;        based on said current vehicle condition controlling said cruise control in order to during vehicle driving towards said first vehicle position increase vehicle speed to said lowest vehicle speed when said vehicle eventually has reached said first vehicle position.        
In a further embodiment of the invention said increase of a vehicle speed to said lowest vehicle speed is only allowed if the difference between said vehicle set target speed and said lowest vehicle speed is below a predetermined value.
The invention also relates to a vehicle cruise control system that comprises (includes, but is not necessarily limited to) a control unit, driver input interface, vehicle position identifying device, road topography identifying device. Said system is characterized in that said control unit is arranged to perform the above mentioned method steps of said first embodiment.